This invention relates to transient suppression circuits comprising metal oxide varistors. More specifically, this invention relates to metal oxide varistor circuits which incorporate thermally responsive elements to achieve time delayed responses.
There are a few known materials which exhibit non-linear resistance characteristics and which require resort to the following equation to quantatively relate current and voltage: EQU I = (V/C).sup..alpha.
where V is the voltage between two points separated by a body of the material under consideration, I is the current flowing between the points, C is a constant, and .alpha. is an exponent greater than 1. Both C and .alpha. are functions of the geometry of the body formed from the material and the composition thereof, and C is primarily a function of the material grain size whereas an .alpha. is primarily a function of the grain boundaries. Materials such as silicon carbide exhibit non-linear or exponential resistance characteristics and are utilized in commercial varistors, however, such non-metallic varistors generally exhibit an alpha exponent of no more than 6.
A new family of varistor materials having alpha exponents in excess of 10 within the current density range of 10.sup.-3 to 10.sup.2 amps/.sup.2 cm has recently been produced from metal oxides. The metal oxide varistor material is a polycrystalline ceramic formed from a particular metal oxide with small quantities of one or more of other metal oxides being added. As one example, the predominant metal oxide is zinc oxide with small quantities of bismuth oxide and other transition metal oxides or post-transistion metal oxides being added. The predominant metal is sintered with the additive oxides to form a ceramic, metal oxide body. Since the metal oxide varistors are fabricated as a ceramic powder, the metal oxide varistor material can be pressed into a variety of shapes of various sizes. Being polycrystalline, the characteristics of metal oxide varistor are determined by the grain size, grain boundary and thickness: all of which can be controlled in a ceramic fabrication process.
The non-linear resistance relationship of metal oxide varistors is such that the resistance is very high (up to a least 10,000 megohms) at very low current levels in the microampere range, and progresses in a non-linear manner to an extremely low value (tenths of an ohm) at high current levels. These non-linear resistance characteristics result in voltage versus current characteristics wherein the voltage is effectively limited, the voltage limiting or clamping action being enhanced at the higher values of the .alpha. exponent. Thus, the voltage versus current characteristics of metal oxide varistors is similar to that of symmetrically bidirectional Zener diodes and covers a greater current range than that of Zener diodes. The conduction mechanism in metal oxide varistors is not yet clearly understood but is completely unlike the avalanche mechanism associated with Zener diodes.
Metal oxide varistors and method of manufacture are described, for example, in U.S. Pat. Nos. 3,496,512 and 3,503,029 to Matsuoka el al., wich are incorporated herein by reference as background material.
Bodies of metal oxide varistors material have commonly been utilized to suppress electrical transients at switch contacts an in power utilization circuits. A comprehensive review of the use of metal oxide varistors for such purposes is described in the Transient Voltage Suppression Manual, edited by David C. Kay, and published by the General Electric Company, Semiconductor Products Department, Syracuse, N.Y., 1976. Specific examples of metal oxide varistor bodies utilized in transient suppression applications are described, for example, in U.S. Pat. Nos. 3,693,053 to Anderson, 3,710,058 to Harnden, 3,710,061 to Harnden, etc.
It has been common to measure power absorbed in metal oxide varistors through additional terminals attached to the face of the varistor body as described, for example in Pat. Nos. 3,742,419 to Martzloff and 3,818,411 to Harnden. The above patents are incorporated by reference herein as background material.